1. Field of the Invention
The present invention relates to paper machines, and, more particularly, to couplings for transmitting rotational power to rolls, cylinders, pulleys and/or gears in paper machines.
2. Description of the Related Art
A paper machine such as a paper-making machine or off-line coater includes a plurality of rotatable driven elements, such as rolls, cylinders, pulleys and/or gears. Such rotatable driven elements are rotatably driven using a drive unit such as an electric motor, pump, etc. It is known to directly or indirectly connect the drive unit with a rotatable driven element using mechanical interconnections such as shafts, chains, belts, etc. It is also known to use a mechanically operated coupling such as a slip clutch with a plurality of pressure plates, etc. Such mechanical clutches have the disadvantage of wearing over time and thus requiring repair and/or replacement.
It is also known to use a magnetic coupling to interconnect a drive unit with a rotatable driven element in a paper machine. The magnetic coupling includes a first rotor carrying a plurality of permanent magnets and a second rotor carrying a plurality of electrical conductors. Rotation of the permanent magnets relative to the electrical conductors causes the generation of counter electro-magnetic fields which provide a rotatable output torque to drive the rotatable driven element. Such a magnetic coupling has the advantage of not wearing over time since the rotors do not physically wear against each other, when compared with a mechanical type clutch.
A disadvantage of a magnetic coupling as described above is that the permanent magnets have a substantially constant magnetic flux, which in turn results in a magnetic field with a substantially constant and finite strength. The efficiency of the magnetic coupling and transmitted torque can be only slightly varied within the operating range of the magnetic coupling.
What is needed in the art is a coupling or clutch which may be used to drive a rotatable driven element in a paper machine which allows the transmitting torque and/or efficiency of the clutch to be varied and controlled.